Previous studies show the classical approach of using only negative feedback in controlling dynamic and steady-state performance of control systems. See, for example, N. K. Sinha, "Control Systems", Holt, Rinehart and Winston, 1986, pp. 59-70, G. H. Hostetter et al, "Design of Feedback Control Systems", Holt, Rinehart and Winston, 1982, pp. 3-5, B. C. Kuo, "Automatic Control Systems", Prentice-Hall, Inc., 4th Ed., 1982, pp. 3-16.
In the field of drive systems including dc drives this classical approach is maintained as seen from B. K. Bose, "Microcomputer Control of Power Electronics and Drives", IEEE Press, 1987.
The ultimate load regulation performance in drive systems should be invariant to the change of load and limited only by the physical properties of the system, such as the finite energy level of available sources, finite energy dissipation capability of available components and finite speed of the transition of control signals. Such an ultimate performance has not been achieved using the classical approach of controlling the drive system with only negative feedback loops.